The subject matter of the present invention relates to a device for catalytically reducing nitrogen oxide (NOx) in the cooling exhaust gases of an internal combustion engine.
When hydrocarbons are burned in an internal combustion engine, such as, for example, in a diesel engine or a spark-ignition engine, not only are the principal combustion products of carbon dioxide and steam formed, but also byproducts, some of which are undesirable. These undesirable byproducts may be nitrogen oxides. The level of nitrogen oxides is dependent not only on the way in which the combustion is carried out but also on the air/fuel ratio. If there are sub-stoichiometric amounts of air, the exhaust gas contains relatively large amounts of carbon dioxide and hydrocarbons. If there is an excess of air the carbon dioxide and the hydrocarbons are almost completely oxidized. The level of nitrogen oxides in an exhaust gas experiences a maximum in the range of a slightly lean mixture composition. However, for internal combustion engines, in particular for spark-ignition engines, an optimum of the specific consumption lies in this range. Therefore, if, in particular, spark-ignition engines are set for an optimally low consumption, it is also possible for the exhaust gas to contain high NOx concentrations.
To reduce the NOx content in an exhaust gas originating from an internal combustion engine, it has become known, for example, from published European application EP 0 487 886 A1 that the selective catalytic reduction of NOx in oxygen-containing exhaust gases is possible using urea and a reducing catalyst. Before it comes into contact with the reducing catalyst, which may form a catalytic converter, the urea is quantitatively hydrolyzed to form ammonia and carbon dioxide. The use of urea has the advantage that there is no need to carry ammonia in motor vehicles and to provide ammonia, which would require appropriate safety precautions.
A problem with the use of urea as reducing agent is that if urea is injected directly upstream of the catalytic converter, undesirable reaction products may form. It is possible for solids to form during the thermolysis of the urea.
To avoid this problem, published German patent application DE 42 03 807 A1 proposes a device for the catalytic reduction of exhaust gases of an internal combustion engine which contain nitrogen oxides. The device has a catalytic converter and a feed device which sprays the liquid urea in finely dispersed form onto a vaporizer that is disposed upstream of the catalytic converter in the prevalent flow direction.
The vaporizer is designed as a flow mixer, with the object that the efficiency of urea decomposition is to be increased. The vaporizer which is designed as a flow mixer and is referred to below as a xe2x80x9cmixerxe2x80x9d, according to DE 42 03 807 A1, is preferably to be designed in such a way that turbulence and radial and/or tangential flow components are generated inside the mixer, so that the heat transfer to the urea solution is completed as quickly and as fully as possible. Furthermore, the intention is to bring about the most uniform possible dispersion of the urea solution and of the gases formed therefrom over the entire cross section of the vaporizer and a hydrolysis catalyst downstream of the vaporizer.
The exhaust gas containing fluid leaving the hydrolysis catalyst, from the internal combustion engine, and ammonia and carbon dioxide are fed to a catalytic converter in which, inter alia, the reduction of nitrogen oxides takes place.
The object of the invention is to provide a device for reducing the nitrogen oxide content in the exhaust gas of an internal combustion engine which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which increases the efficiency of the device for the catalytic reduction of exhaust gases from an internal combustion engine which contain nitrogen oxides still further.
With the above and other objects in view there is provided, in accordance with the invention, a device for catalytically reducing NOx-containing exhaust gases of an internal combustion engine, comprising:
an exhaust pipe for receiving and conducting exhaust gas of an internal combustion engine in a given flow direction;
a feed device communicating with the exhaust pipe for introducing a reducing agent, such as urea, into the exhaust pipe;
at least one mixer disposed downstream of a point at which the reducing agent is fed into the exhaust pipe in the given flow direction, the mixer being formed with a multiplicity of flow passages;
at least one catalytic converter disposed downstream of the mixer in the given flow direction; and
a diffuser disposed between the mixer and the catalytic converter, the diffuser being formed with a multiplicity of passages through which the exhaust gas can flow, the passages of the diffuser having a cross section increasing in the given flow direction.
In other words, the device according to the invention for the catalytic reduction of NOx-containing exhaust gases from an internal combustion engine has an exhaust pipe, one end of which can be connected to at least one exhaust outlet from an internal combustion engine. A feed device is provided, by means of which a reducing agent, in particular urea, can be introduced into the exhaust pipe. Furthermore, the device has at least one mixer which is arranged downstream of the point where the reducing agent is fed into the exhaust pipe, as seen in the direction of flow of the exhaust gas, this mixer having a multiplicity of passages through which the fluid can flow. At least one catalytic converter is arranged downstream of the at least one mixer. The device according to the invention is distinguished by the fact that between the mixer and the converter there is at least one diffuser which has a multiplicity of passages through which the exhaust gas can flow and which have a cross section which increases in the direction of flow.
This inventive configuration of the device ensures that the fluid leaving the at least one mixer, which contains the internal combustion engine exhaust gas and the decomposition products formed from the reducing agentxe2x80x94in particular ammonia and carbon dioxidexe2x80x94is transferred uniformly into the catalytic converter. This ensures that the fluid flows onto the catalytic converter relatively uniformly over its entire cross section, so that the catalytic converter is uniformly utilized.
By means of the diffuser, which has a multiplicity of passages through which the exhaust gas can flow and which have a cross section which increases in the direction of flow, it is possible to disperse the fluid very uniformly over the entire entry side of the catalytic converter. The pressure drops in the diffuser are relatively low.
In accordance with an added feature of the invention, the mixer has an outlet with a defined cross section and the diffuser has an inlet with a free cross section of flow at least as large as the cross section of the outlet of the mixer. This ensures that the pressure drops in the transitional region between the mixer and the diffuser are relatively low.
In accordance with an additional feature of the invention, the diffuser at the inlet has a cell density which corresponds to a cell density of the mixer. In this way, further pressure drops are avoided. Cell density is defined as the number of passages per cross-section unit area.
In accordance with another feature of the invention, a cross-sectional shape of the passages of the diffuser at an inlet of the diffuser corresponds substantially to a cross-sectional shape of the passages at an outlet of the mixer.
In accordance with a further feature of the invention, at least some of the passages at the inlet of the diffuser are aligned with corresponding the passages at the outlet of the mixer.
In other words, the cross-sectional shape of the individual passages at the entry to the diffuser corresponds substantially to the cross-sectional shape of the individual passages of the mixer at the exit. Preferably, at least some of the individual passages of the diffuser at the entry are aligned with some of the individual passages of the mixer at the exit. This arrangement and design of the mixer and the diffuser ensures that the passage of the mixer merges, as it were, into the passage of the diffuser. As a result, pressure drops in the transitional region between diffuser and mixer are reduced still further.
To improve the mixing of the urea with the exhaust gas still further, it is proposed for the passages of the diffuser to be offset with respect to the passages of the mixer at the exit. As a result, a filament of flow which emerges from a passage of the mixer is broken up into a plurality of partial flows at the entry to the diffuser. These partial flows are mixed with further partial flows which are formed by splitting other filaments of flow which emerge from the mixer.
Since the passages inside the diffuser increase in size in the direction of flow, it is also ensured that the diffuser has a certain mixing action on the fluid inside a passage, since the flow velocity inside the diffuser is reduced, so that the ratio between flow velocity and diffusion rate shifts in the direction of the diffusion rate.
In accordance with again an added feature of the invention, the diffuser is formed with a jacket casing and at least one stack of smooth and undulating sheet-metal layers which are layered on top of one another. The configuration of the diffuser may correspond to the diffuser structure disclosed in the international PCT publication WO 89/02978 or the publication WO 93/20339. The disclosures of those publications is herewith incorporated by reference.
To increase the mixing of the fluid still further, it is proposed, according to yet another advantageous configuration, for at least some of the passages of the diffuser to be connected to one another in terms of flow.
The passages of the diffuser are delimited by passage walls. Preferably, at least some of the passage walls have openings. In particular, it is proposed for at least some of the openings to be assigned guide surfaces which run obliquely with respect to the passage wall. These guide surfaces divert the flow from one passage into an adjacent passage.
To achieve the quickest possible mixing of the fluid in the radial direction of the diffuser, it is proposed for smooth sheet-metal layers, which are at least partially of mesh-like design, to be provided between undulating sheet-metal layers. The smooth sheet-metal layers may be formed by an expanded metal.
In accordance with a concomitant feature of the invention, the diffuser is disposed substantially directly and immediately behind the mixer.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device for reducing the level of nitrogen oxides in an exhaust gas from an internal combustion engine, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.